In an ideal television system the light output produced by a kinescope would be linearly related to the light applied to a camera pick-up tube. However, in practical systems, the camera tube and the display tube are both non-linear devices, and the overall camera/display net "gamma" (which represents the curvature of the transfer function) differs from a value of unity which would be necessary for linear response.
The non-linearities, if uncorrected, would tend to reduce the contrast and to make detail in dark picture areas almost invisible while the whites would tend to be amplified to the point of picture tube saturation and blooming. Correction of non-linearities (i.e., gamma correction) requires non-linear processing of the video signal to be displayed.